We STEPHEN TABER 
of which is maintained by diffusion from without. When a crys- 
tal grows in a direction in which growth is opposed by external 
pressure, the pressure is transmitted through a thin layer of solu- 
tion separating the crystal from the foreign body. The effect of 
pressure and of capillarity, if the latter be present, is to reduce the 
thickness of this layer to a minimum; but it would be difficult, if 
not impossible, to completely expel it by pressure alone from 
between two smooth parallel surfaces. And crystal growth would 
tend to make the surfaces under pressure parallel, for deposition 
would be most rapid where diffusion is least restricted, i.e., where the 
layer of solution is thickest. Therefore a crystal growing in a 
limited space may make room for itself by forcibly enlarging this 
space, if it is supplied with the material for growth by diffusion 
through solutions occupying spaces that are sufficiently small. 
The solubility of most substances, including calcite, is increased 
by pressure, and when such a substance separates from solution, 
there is an increase in volume which may result in pressures greatly 
exceeding the crushing strength of the crystals, provided the solu-_ 
tion cannot readily escape. If the material that incloses a growing 
crystal is rendered more soluble by pressure, it may be gradually 
removed in solution as the crystals are enlarged. This probably 
explains the replacement of limestone and vein calcite by the idio- 
morphic crystals of pyrite. 
The tendency of a crystal to assume a regular polyhedral form 
is important as a factor in the development of pressure during 
crystal growth only in so far as it affects the relative solubility of 
the crystal in different directions. While the difference in the pres- 
sure that may be developed in any two directions during the growth 
of a crystal is probably small, it can accomplish appreciable results 
if continued through a long enough period of time. 
SUMMARY AND CONCLUSIONS 
The small and relatively simple veins of a region of unaltered 
sedimentary rock were studied in order to obtain field evidence 
bearing on the mechanics of vein formation. ‘Two types of veins 
are described, one fibrous and the other coarsely crystalline and 
non-fibrous. Both consist essentially of calcite or of gypsum. 
